You can use Spond without having the Spond app. If you register with an email and dont download the app, notifications will be sent to your email and you can respond from here. You can also log in on https://spond.com/login to get a better overview of the whole group.
If you have already downloaded the app but now only want to use email, you can log out of the app to start receiving email notifications instead. As soon as you log back into the app, the notifications will come to your phone again.